FIG. 6 shows a conventional drive circuit disclosed in U.S. Pat. No. 5,552,746 corresponding to JP-A-H8-293774. The drive circuit receives a control signal and outputs a drive signal to a voltage-driven semiconductor element (i.e., transistor) 144 according to the control signal. The drive circuit includes an active voltage clamp circuit that protects the gate of the transistor 144 from an excessive electrical stress. The clamp circuit includes Zener diodes 132, 134 and a current mirror constructed with transistors 136, 138. If a power supply +B is less than a threshold voltage Vth0, the active voltage clamp circuit is passive and does not affect the operation of the drive circuit. The threshold voltage Vth0 is given as follows:Vth0=Vth1 of the diode 132+Vth2 of the diode 134+VBE of the transistor 138
Conversely, the active voltage clamp circuit becomes active when the power supply +B is at or above the threshold voltage Vth0 by conducting an electric current through the diodes 132, 134 and the transistor 138. The current flowing through the transistor 138 is mirrored through the transistor 136 which is connected to the gate of the transistor 125. If the power supply +B is at a voltage larger than the threshold voltages Vth1, Vth2 of the diodes 132, 134, an electric current will begin to flow through the diodes 132, 134 and that current will be mirrored through the transistor 136 which would then pull the gate of transistor 125 low. Consequently, the gate potential of transistor 144 is clamped at the voltage threshold of diodes 132, 134 plus the voltage drop across transistor 138. Thus, the current flow is limited by the feedback loop established through the diodes 132, 134 and the current mirror constructed with the transistors 136, 138.
In the above-described conventional drive circuit, the transistor 140 at the output stage is connected in an emitter follower configuration so that the gate potential of the transistor 144 is clamped by the base-emitter voltage VBE. However, the gate potential cannot exceed a level that is obtained by subtracting the base-emitter voltage VBE from the power supply +B. Therefore, if the power supply +B decreases, a bias voltage enough to adequately drive the transistor 144 cannot be produced.